This invention relates to a casing. More particularly, it relates to a casing for a borehole in the ground, in which an annulus is formed between the casing and the wall of the borehole, and in which the casing is provided with at least two packer groups, each including activatable packers, the packers being formed, in their active state, to seal the annulus. At least one packer group of the activatable packers is connected to sliding sleeves, so that at least one group of packers is movable relative to the casing.
In drilling in the ground, as it is known from the recovery of petroleum, among other things, it is usual to run a casing into the ground. The main purposes of the casing are to stabilize the borehole and prevent liquid flow between the different zones of the well. For the latter to be achieved, the annulus between the casing and borehole is often filled with cement.
According to the prior art, drilling and the running-in of casing are most often carried out as separate operations, but it is also known to use a method in which a casing is inserted into the borehole during the drilling operation itself.
To penetrate, for example, a petroleum-bearing formation which is located relatively far from the drilling rig, and which may also extend over a considerable distance, it has become usual to drill approximately horizontal boreholes up to several kilometres long. These boreholes may also extend through several petroleum-bearing zones.
If the horizontal part of the well exceeds a certain length, it has turned out to be difficult to place casings into the well because of increasing frictional force in long horizontal wells. The weight of the part of the casing located in the approximately vertical part of the borehole may then be insufficient for moving the casing further into the approximately horizontal part of the borehole.
When drilling through more than one zone, or through a long zone, it can also be a challenge to stabilize the well in terms of pressure, as the different zones, or different parts of a long zone, may exhibit different pressures while, at the same time, it must be possible to supply sufficient drilling fluid to the borehole to ensure conveyance of cuttings via the annulus out of the borehole.
Thus, it may happen that drilling fluid is entering the formation in one zone, thereby damaging the formation with respect to subsequent production, while formation fluid is flowing out of another zone at the same time. The latter entails considerable drilling-technical problems. At worst the circumstance may result in an uncontrolled blow-out from the well and can also contribute to an uncased borehole wall caving in.
In shorter wells such problems are normally remedied by the cementation of a casing to the well wall in the problematic zone, and the well is drilled on from there with a somewhat smaller diameter than before. However, only a limited number of casings of successively smaller diameters can be placed in the well in this way. In relatively long wells this solution will often not be usable.
The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art.